Field of the Invention
The present invention relates to an optical head apparatus capable of recording/reproducing information from an optical disc.
Hitherto, a technology for forming an optical head apparatus by a thin layer has been proposed, the optical head being capable of recording/reproducing information from an optical disc. For example, there has been a technology disclosed in PCT/JP 88/01344. FIG. 1 is a view which illustrates the structure of the conventional thin-film optical head apparatus. Referring to FIG. 1, a transparent substrate 2S holds a hollow Si-substrate 1 therebetween, the transparent substrate 2S having the surface on which gratings 4B and 4C are formed. The transparent substrate 2S is made of, for example, thermosetting resin. The concavities and convexities formed on the surface of the gratings 4B and 4C are formed by transferring the grating of a stamper. The grating 4C is formed in a circular region whose central axis is arranged to be an axis L passing through a point O, the grating 4C being in the form of a concentric or spiral cyclic concave and convex structure formed around the central axis L. The gratings 4B and 4C do not overlap each other and annular gaps are present therebetween. Waveguide layers 3B and 3C are formed on the transparent substrate 2S with a transparent layer 5 having a low refractive index formed therebetween, the waveguide layers 3B and 3C respectively having a higher refractive index than that of the transparent layer 5. Since the thickness of the transparent layer 5 is thin, the concavities and convexities of the gratings on the transparent substrate 2S remain on the transparent layer 5. The waveguide layers 3B and 3C formed thereon with concavities and convexities serve as grating couplers. The refractive index of the waveguide layer 3B is larger than that of the waveguide layer 3C, but the film thickness of the waveguide layer 3B is smaller than that of the waveguide layer 3C. The waveguide layer 3B is formed in an annular region relative to the central axis L, while the waveguide layer 3C is formed in a circular region relative to the central axis L. The inner peripheral portion of the waveguide layer 3B and the outer peripheral portion of the waveguide layer 3C overlap each other in such a manner that the waveguide layer 3C covers the waveguide layer 3B, the overlapped portion being positioned in an annular region positioned between the grating coupler 4C and the grating coupler 4B. A ring-like photo detector 6 is formed on the substrate 1 at the innermost portion of the waveguide layer 3B.
Laser beams emitted from the semiconductor laser 8 are turned into parallel ray light by a converging lens 9. The laser beam 11 converted into concentric polarization (a state of polarization in which a plane of vibration is tangent to a circle) or radial polarization (a state of polarization in which a plane of polarization is tangent to a circle) by a quarter wavelength plate 10C and a polarizing element 10, is made incident upon and coupled by the grating coupler 4C so as to be waveguide light 12C in a TE mode or a TM mode, the waveguide light 12C being propagated radially in the waveguide layer 3C. The waveguide light 12C is converted into waveguide light 12B after it has been shifted from the waveguide layers 3C and 3B, the waveguide light 12B being then converted into radiation mode light 13 which is converged at a point F on the central axis after being radiated from the grating coupler 4B. A reflection surface 16 of the optical disc is orthogonal to the axis L, the reflection surface 16 being positioned at substantially the same position as the focal point F. Therefore, the light is reflected by the reflection surface 16 so as to become reflected light 17B, the reflected light 17B being then made incident upon and coupled by the grating coupler 4B. As a result, the light is converted into waveguide light 18B which moves toward the center of the waveguide layer 3B. The thus formed waveguide light 18B is radiated at the innermost position of the waveguide layer 3B, and the quantity of the light is detected by the photo detector 6 disposed on the substrate 1.
FIG. 2 illustrates the detection of a signal performed by a conventional example. The grating coupler 4B is divided into 6 regions (that is, 4B1, 4B2, 4B3, 4B4, 4B5 and 4B6) by three straight lines passing through the center O. The regions 4B4, 4B5 and 4B6 are positioned diagonally to the corresponding regions 4B1, 4B2 and 4B3 with respect to the center O, the regions of the grating which are positioned diagonally relative to each other being in the form of the same shape (the same concave and convex structure). The radiated beams from the grating couplers 4B1 and 4B4 are converged at FC, while the radiated beams from the grating couplers 4B2 and 4B5 and that from the grating couplers 4B3 and 4B6 are converged to corresponding points FA and FB on the central axis L. The point FC is positioned at substantially a midpoint between the points FA and FB. The photo detector 6 is divided into 8 regions (that is, 6B1, 6B2, 6B3, 6B4, 6B4', 6B5, 6B6, and 6B1') by 4 straight lines passing through the center O. The regions 6B1, 6B1', 6B4 and 6B4' are respectively divided in the direction of rotation of the optical disc so as to confront the inner surfaces of the grating couplers 4B1 and 4B4. As a result, the photo detector 6 detects the quantity of returned waveguide light which has been supplied and coupled by the grating couplers 4B1 and 4B4. Also the regions 6B2, 6B3, 6B5 and 6B6 are respectively arranged to confront the inner surfaces of the regions 4B2, 4B3, 4B5 and 4B6 so that the quantity of waveguide light which has been supplied and coupled by the corresponding couplers is detected. A sum signal of 6B1 and 6B4 and that of 6B1' and 6B4' are derived by adding amplifiers 100 and 101, while the differences between the above regions are derived by a differential amplifier 102 so that a TE signal 103 is created. On the other hand, a reproduced signal 105 is created by conducting additions by an adding amplifier 104. Similarly, a sum signal of 6B2 and 6B5 and that of 6B3 and 6B6 are derived by adding amplifies 106 and 107, while the differences between the above regions are derived by a differential amplifier 108 so that an FE signal 109 is created.
FIG. 3 illustrates a change in the incident angle of reflected light. A grating pitch .LAMBDA. at an emergent position A of radiated light can be obtained as a function of a diameter r from the following equation: EQU .LAMBDA.=.lambda./(N+r/(f.sup.2 +r.sup.2).sup.1/2) (1)
That is, the convergency toward the focal point F is realized by using the grating pitch .LAMBDA. as a function of the diameter r, where symbol .lambda. represents the wavelength of a laser beam, N represents an equivalent refractive index of the waveguide passage, and f represents a focal length {however, since the surface of the optical disc on which light is reflected is generally covered with a transparent plate, correction is necessary for a spherical aberration which can be generated when a converged light transmits through a parallel plate, that is, it is required to add a correction term to Equation (1)}. A diffraction angle .theta. of the radiation-mode light 13 can be given from the following equation: EQU sin .theta.=.lambda./.LAMBDA.-N (2)
The factors q, r, and f hold the following relationship: EQU tan .theta.=r/f (3)
When the reflection surface 16 is positioned at the focal position F, light is reflected passing through AFA', while light is reflected passing through ABC when the reflection surface 16 is positioned closer to the focal point F by a distance .epsilon. (or positioned away from the focal point F by the same distance). Although light made incident upon point C on the grating coupler in a direction FC can be efficiently converted into waveguide light, its conversion efficiency (input coupling efficiency) deteriorates in proportion to the deflection from the direction FC. An angle (.theta.-.theta.') of deflection with respect to the direction FC can be approximately obtained from the following Equation: EQU .theta.-.theta.'=tan.sup.-1 (2.epsilon.r/f.sup.2 +r.sup.2))(4)
Therefore, the deflection angle (.theta.-.theta.') increases as .epsilon. increases causing the conversion efficiency to deteriorate. In order to make different from each other the points at which the radiated light from the grating couplers 4B1, 4B2 and 4B3 is converged respectively, there is a method of designing different grating couplers which correspond to different focal lengths f, the design being conducted in accordance with Equation (1). According to Equation (2), the focal points can be differ from each other by changing equivalent refractive index N for a waveguide passage in each of the coupler regions by using the same grating. For example, an increase in the thickness of the waveguide layer in a sequential order as 4B2, 4B1 and 4B3 causes the equivalent refractive index N to be increased in this order. Therefore, the focal points can differ easily. There is another method of varying the equivalent refractive index in which materials having different refractive indexes are placed on one waveguide layer.
FIG. 4(a) illustrates variation in the input coupling efficiency of reflected light as the position of the reflection surface varies. FIG. 4(b) illustrates, variation in the output of an FE signal, as the position of the reflection surface varies.
As is shown from Equation (4), the input coupling efficiency 19A with which the conversion into waveguide light is made by the grating couplers 4B2 and 4B5 varies depending upon the position of the reflection surface, the input coupling efficiency 19A describes a curve having its maximal value when the reflection surface is at the position FA. Similarly, the coupling efficiency 19B with which the conversion into waveguide light is carried out by the grating couplers 4B3 and 4B6 describes a curve having its maximal value when the reflection surface is at the position FB. Since the coupling efficiency is directly proportional to the quantity of returned waveguide light, the FE signal 109 has an S-curve characteristic with respect to the defocus of the reflection surface as shown in FIG. 4B. Therefore, a focus control can be conducted.
Furthermore, the zero-cross point of the output of the FE signal 109 corresponds to the case where the reflection surface is at the position FC. At this time, the coupling efficiency 19C in conversion into waveguide light by the grating couplers 4B1 and 4B4 is the largest. Therefore, when the reflection surface is positioned at the position FC, the influence from the signal (pits or dots) on the reflection surface of the optical disc appears in the form of a change in the optical distribution of reflected light 17B or a change in the quantity of light, that is, the change in the quantity of returned waveguide light 18B. As a result, high quality reproduced signal can be derived from a signal 105.
On the other hand, the reflection surface 16 of the optical disc has radially periodical guide grooves or pits in the direction of the rotation of the disc so that a tracking error appears in the form of an unbalance of the quantity of reflected light 17B in the radial direction of the disc when the reflection surface is positioned at the position FC. Therefore, when the quantity of light of waveguide light 18B is equally divided by a division line drawn in the direction of the rotation of the disc and the difference in the quantities of light is derived, the TE signal can be obtained from the signal 103 based on a so-called a push-pull method.
FIG. 5 illustrates a principle for converting linear polarization into concentric polarization. Linearly polarized laser beams which have been turned into parallel light by the converging lens 9 are converted into circularly polarized light 11' (states of polarization 11A', 11B', 11C' and 11D') after they have passed through the quarter wavelength plate 10C. The polarizing element 10 is formed by homogeneous type liquid crystal elements in such a manner that a homogeneous type liquid crystal 24 is disposed between transparent substrates 22 and 23. The surfaces 25 and 26 of the transparent substrates 22 and 23 are rubbed in directions (directions 24A, 24B, 24C and 24D) inclined by an angle of 45.degree. from tangents to the concentric circle around the center O. Also the liquid crystal 24 is oriented in the same direction. Therefore, circularly polarized light 11' can be converted into radially or concentrically polarized light 11 by designing the liquid crystal 24 in such a manner that a component of light which transmits the liquid crystal 24 in the direction of the orientation delays (or advances) by a quarter wavelength. At this time aberration (phase lag) caused by polarizing conversion is corrected by adjusting the thickness of the transparent substrate 22 or 23, and accordingly, conversion into eccentric polarization or into radial polarization can be made completely (with no aberration). Thus, the following explanation is based upon this complete polarization. In the case of a concentrically polarized light, the directions of the polarization become 11A, 11B, 11C and 11D. When concentrically polarized light has been transmitted and coupled in the waveguide layer 3C by the grating coupler 4C, its waveguide light 12C is converted into the TE mode, while radially polarized light is converted into the TM mode. On the other hand, when waveguide light 12C, that is, 12B is in the TE mode, the radiated light from the grating coupler 4B is concentrically polarized light. When it is in the TM mode, the radiated light is radially polarized light. The structure may be constituted without the polarizing element 10. In this state, when a grating pitch .LAMBDA. of the grating coupler 4C has been designed for coupling in the TE mode (that is, it has been designed as a solution of Equation (1) for the equivalent refractive index of the waveguide light in the TE mode), waveguide light 12C is in the TE mode, while the same is in the TM mode when the grating pitch .LAMBDA. has been designed for coupling in the TM mode.
However, the optical head apparatus of the type described above arises the following problems:
First, the convergence of concentrically or radially polarized light 13 radiated from the grating coupler 4B is inferior. A curve (a) shown in FIG. 9 shows a cross section of the intensity distribution of concentrically polarized light (or radially polarized light) on a reflection surface {coordinate (.xi., .eta.)}. That is, the field vectors of concentrically polarized (or radially polarized) light positioned diagonally with respect to the center are opposite to each other. Therefore, the field vectors are cancelled at the convergent point, causing the convergence to deteriorate.
Second, since the grating coupler 4B is arranged to be a ring-like shape, reflected light from the optical disc is shifted excessively from the ring-like portion when the optical disc has a reflection surface which is likely to diffract (scatter) light easily. It leads to a fact that the quantity of light which can be converted into waveguide light by the grating coupler 4B is reduced and the quality (C/N or the like) of the reproduced signals or control signals deteriorate.
Third, since the signal reproduction is achieved by increasing/decreasing the quantity of reflected light and input coupling light, a signal reproduction utilizing a change in a magnetic signal, that is, a signal reproduction utilizing the polarization state of light cannot be conducted.
Fourth, the output of the TE signal based on the push-pull method is the largest when the depth of pits is a 1/8 wavelength, while the same is zero when the depth is 1/4 wavelength. Therefore, the detection of TE signals depending upon the conventional push-pull method cannot be conducted in a compact disc (CD) whose depth of pits is substantially 1/5 wavelength.